In polypolar polyphase type linear d.c. brushless motors, the polarity of each magnetic pole in a field magnet is discriminated by a pole-discriminating sensor to suitably change over the direction of an electric current fed to armature coils, thereby causing a moving element to travel linearly. The linear d.c. brushless motors capable of travelling linearly as described above have an advantage that the moving stroke of the moving element can be set up very long. However, such motors require a long linear guide mechanism for supporting the moving element so as to be movable linearly and reciprocally over the whole stroke. One of demerits of the linear d.c. motors when compared with rotating d.c. motors is that they must use a very expensive linear guide mechanism.
In general, a coreless type linear d.c. motor can use a relatively cheap linear guide mechanism because the magnetic attraction force [vertical force] between its moving element and stator is so small that it can be disregarded owing to its coreless structure, compared with a linear pulse motor and a core type linear d.c. motor.
Therefore, it has heretofore been attempted to fabricate a coreless and polypolar polyphase type linear d.c. motor LDM, as illustrated in FIGS. 19 through 21, by using a stator yoke 21, which can be incorporated at an extremely low cost, as a constituent element for linear guide without using any particular linear guide mechanism.
The conventional polypolar polyphase and moving-magnet type linear d.c. brushless motor LDM will be described with reference to FIGS. 19 through 21. The linear d.c. brushless motor LDM has such a structure that a group of air-core type armature coils 8 is disposed on the upper surface of the stator yoke 21 to compose a coreless stator armature 9, and a polypolar field magnet 10 formed by contiguously and alternately arranging magnetic poles, which have each an N or S polarity and a width of T as shown in FIG. 22, through a minute air gap over the coreless stator armature 9 is used as a moving element 24.
Incidentally, numeral 21 indicates a stator yoke-cum-linear guide, and numeral 11 designates a printed-wiring board arranged on the surface of the coreless stator armature 9, which faces the field magnet 10, and composed of a non-magnetic material. On the printed-wiring board 11, is formed a printed-wiring pattern (not shown) for electrically connecting terminals of pole-discriminating sensors 12 and the group of the armature coils 8. Numeral 22 indicates a traveling yoke. On both sides thereof, extended bent parts 22a extending downward are formed. Guide rollers 23 rotatably attached to the extended bent parts 22a are caused to guide along both sides of the stator yoke-cum-linear guide 21 to move them, whereby a moving element 24 composed of the traveling yoke 22 with the field magnet 10 attached thereto is linearly moved relative to a stator 25 composed of the coreless stator armature 9 and the like. Numeral 26 designates a stationary base for the stator 25.
Such a linear d.c. brushless motor LDM can save using an expensive linear guide because the stator yoke 21 itself, which is used to shut the magnetic path of a magnetic circuit, serves as a constituent element for linear guide mechanism, and hence has an advantage that it can be fabricated at a very low cost.
However, the linear d.c. brushless motor LDM involves a potential problem that when it is fabricated as a high-thrust motor or it is run at a high speed, the guide rollers 23 are derailed from the sides of the stator yoke 21 due to vibrations generating at that time. The linear d.c. brushless motor LDM making use of such a stator yoke 21 has also been accompanied by a demerit that since it has a structure that the moving element 24 tends to detach from the stator 25, its installing manner is limited, resulting in a motor having limited applications due to such a structure.